There have been known image forming apparatuses which use a developing method of the so-called contact type, that is, image forming apparatuses which form an electrostatic latent image on their rotatable image bearing member, and develop the electrostatic latent image by placing their development roller in contact with the peripheral surface of the image bearing member. It is possible that the usage of the developing method of the contact type will damage the peripheral surface of an image bearing member. Therefore, in order to prevent the peripheral surface of an image bearing member from being damaged by a development roller, many image forming apparatuses, which use the developing method of the contact type, employ an elastic development roller, which comprises: a metallic core; and an elastic surface layer which covers the entirety of the peripheral surface of the metallic core. The elastic development roller, however, suffers from the following problem. That is, the elastic surface layer of an elastic development roller is in contact with the peripheral surface of an image bearing member, and therefore, is gradually shaved by the peripheral surface of the image bearing member. Thus, the fine particles which result from the shaving of the peripheral surface of the elastic layer of a development roller travel to the cleaning edge of a cleaning blade, causing thereby the cleaning blade to shudder, chatter, and/or lap, which in turn generates abnormal sounds, makes the peripheral surface of the image bearing member unsatisfactorily cleaned, and/or causes the like problem. In other words, as the peripheral surface of a development roller is shaved by the peripheral surface of a photosensitive member, it becomes difficult for the image forming apparatus to output images which are satisfactory to users. In terms of the lengthwise direction of a development roller, it is likely to be across the end portions (portions which are not supplied with toner) that the elastic layer of a development roller is shaved. Thus, there have been proposed various arts for minimizing the amount by which the elastic layer of the development roller of an image forming apparatus which uses a developing method of the contact type is shaved. Some of these arts have been disclosed in Japanese Laid-open Patent Applications S52-143831, and H01-191880 (Patent Documents 1 and 2, respectively).
The above-mentioned prior arts, however, suffer from the following problems. That is, according to these arts, in order to prevent the elastic layer of a development roller from being shaved, the elastic layer is to be uniformly increased in hardness across the entirety in terms of the lengthwise direction of the development roller. However, the increase in the hardness of the elastic layer of a development roller increases the amount of the pressure to which the toner on the peripheral surface of the elastic layer of the development roller is subjected, which in turn accelerates toner deterioration. Toner deterioration causes an image forming apparatus to output unsatisfactory images, in particular, foggy images.
According to the art disclosed in the second patent document, the elastic layer is formed of foamed material, and the entirety of its peripheral surface is covered with a piece of seamless tube. This art also is problematic in that it substantially increases a development roller in manufacture cost.